feat(content): bake the Jardin des Formes terrain showcase site

Five reference landforms on one 512 m field at 1 m per cell: a stratified
canyon through a mesa, a crevasse between two buttes, a boulder field on
rippled sand, two rock spurs framing a sand channel, and a sinuous
escarpment with knolls on the slope below.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/briefs/captures/terrain_showcase_terrain_preview.png.import

@@ -0,0 +1,40 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://c448uwrxq3foi"
+path="res://.godot/imported/terrain_showcase_terrain_preview.png-6c9dafd8892db2b24553477d0b6fc653.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://docs/briefs/captures/terrain_showcase_terrain_preview.png"
+dest_files=["res://.godot/imported/terrain_showcase_terrain_preview.png-6c9dafd8892db2b24553477d0b6fc653.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/uastc_level=0
+compress/rdo_quality_loss=0.0
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/channel_remap/red=0
+process/channel_remap/green=1
+process/channel_remap/blue=2
+process/channel_remap/alpha=3
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1